This invention relates to the fabrication of composite structures, and more particularly to the fabrication of hollow structures having non-circular cross sections.
Composite materials are widely used in the field of structural applications. Further, nonconducting composite beams have found particular use in boom trucksxe2x80x94trucks that are used to lift a bucket or similar structure containing a person and/or machinery to an elevated position. Typically trucks of this variety are used for building maintenance, repair/installation of high voltage wires, and similar activities. For such uses and many others, the beam must be capable of withstanding considerable loads and stresses. Also, when used as a boom for high voltage repair and/or installation, the beam must have a high degree in dielectric properties, such that the beam is able to withstand a very high voltage while allowing a very low electric current to pass therethrough.
The filament winding process is often employed in the manufacture of composite beams. In addition, it is often desirable to make rectangular shaped beamsxe2x80x94typically, square beamsxe2x80x94as well as other forms having flat as opposed to rounded sides. In a typical winding operation, a mandrel of the desired shape of the end resulting beam is utilized. A bundle of fibres, generally impregnated with a resin, are wound along the length of the mandrel in a pattern dictated by the winding angle, which is the angle the fibre makes with the longitudinal axis of the mandrel. This angle is, in turn, dictated by the structural requirements of the end product.
Once the desired thickness for the beam walls is achieved, the mandrel overwound with the composite material is left to cure at room temperaturexe2x80x94or in some cases at an elevated temperature depending on the resin. Once cured, the mandrel is removed, and the composite beam structure is ready for use. However, the rectangular tubes made from conventional filament winding process have disadvantages compared with round tubes made from the same process.
In the case of round tube winding, the tension force developed in the resin-impregnated fibres during the winding process maintains a uniform radial pressure on the surface of the tube. As the tension force increases, more radial pressure develops, which squeezes the resin out; and that results in a higher fibre content in the finished laminate. The fibre content in the filament wound round tube can be controlled uniformly throughout the laminate. Fibre content is defined as the percentage of fibre by weight as measured in the cured composite beam. In addition, the wall thickness will be consistent as a result of the uniform radial pressure.
However, in rectangular tube winding, the tangential force arising from the fibre tension against the flat surface is much smaller than in the case of a round tube. Furthermore, the pressure does not develop uniformly. At each corner of a rectangular tube, there will be a higher radial pressure. This uneven distribution of pressure creates an uneven fibre content and inconsistent wall thickness. A normal rectangular filament wound beam typically has rounded corners and exhibits a greater wall thickness toward the central portion of the wall on each side of the rectangle, and a much smaller wall thickness towards the corners. A great many remedies have been explored in an attempt to achieve uniform wall thickness.
In the past, generally speaking, there have been two methods to control the external dimensions of a rectangular beam. First, by grinding the crown down to a flattened state, or second, by pressing the crown down to a flattened state in the curing process. Each method encounters the problem of losing strength due to the cutting of continuous fibres and/or the loosening of the tension that was created in the fibres through the winding process. In each case, the final product has a large corner radius with a thinner wall thickness at each of the corners.
The difficulty associated with inconsistent wall thickness is how that inconsistency affects the beam strength. In beams, especially those used as booms such that they act in essence as a cantilever beam, it is important to ensure high bending strength. In turn, it is also important that the beams have a relatively high shear strength. The required strength of each beam will change depending on the intended end use. In turn, the overall strength may be varied by adjusting the winding angle.
In general, when a beam is supported at least at one end and a load is applied to the other end, it bends. In this simple cantilever bending the beam experiences various types of stresses, their extent depending on the location in the beam. The most critical points in the beam are the bottommost corner points. At these points, compressive stress combined with shear stress is at a maximum in the beam. Thus, the strength at these corners is very important, more so than other areas in the beam. If the wall thickness is thinner at these points, higher stresses develop and may result in failure at early stages of loading. In addition, the wall thickness at these corners influences the beam stability against elastic buckling of the bottom wall which is subject to a compressive stress.
There is a need for an improved approach to the fabrication of a composite structure having a non-circular cross section.
U.S. Pat. No. 5,238,716 issued to Adachi teaches a composite beam having a hollow cross-section. The beam is made of three distinct structural layers. An inner structural layer comprises a glass filament winding, a middle structural layer comprises four plates placed around the inner structural layer, and an outer structural layer comprises layers of woven and nonwoven fibre material. Each of the inner and outer structural layers have been saturated with resin. Pressure is applied to the second structural layer to remove all entrapped air. The beam has very high compressive and tensile strength. The difficulty in manufacturing such a beam is the requirement for three separate processes to form the three distinct structural layers, making the procedure time consuming and costly.
U.S. Pat. No. 5,505,030 issued to Michalcewiz et al teaches a composite reinforced structure. The reinforced load supporting structure has an inner load supporting structure and an outer exo-skeleton. The inner load supporting structure, typically a column or beam, is enclosed by a layer of at least one distinct piece of preformed engineering material having both a high tensile strength and high modulus of elasticity which forms the exo-skeleton. The exo-skeleton is used to reinforce the load supporting structure and reinforce areas that typically suffer from cracking. An adhesive substance adheres each of the layers (when there is more than one) of the exo-skeleton to the next. The invention allows for reinforcement of generally any size or shape of object. However, the increase in tensile strength of the beam is derived only from the outer exo-skeleton applied to the inner load supporting structure.
U.S. Pat. No. 5,549,947 issued to Quigley et al teaches both the structure and manufacture of a composite shaft. The composite member has a plurality of plies; an interior ply functioning to dampen sudden forces; an intermediate ply functioning as a load carrier; and an exterior ply being abrasion resistant. At least one of the plies has a biaxial or triaxially braided fibre geometry, where one or more of the fibres are helically wound about the circumference of the composite member. The total bending stiffness of the composite member is divided amongst the three plies. The second ply has a share of the bending stiffness that is greater than either of the first or third plies shares. The present invention is intended primarily for tubular shafts and provides a light weight, high strength composite member.
U.S. Pat. No. 5,688,571 issued to Quigley et al is a continuation in part of the previously described patent. This patent teaches a tubular member having a plurality of plies and a method of fabricating and incorporating side by side closed tubes, typically having a circular or rectangular cross-section. The composite member is generally formed by a continuous sheet of fabric. Elongated strips of fabric are formed into side by side closed tubes. A ply of axially extending fibres is then disposed over the tubes. The fabric is generally a preformed fabric, preferably nonwoven. A resin is applied to the plies of fabric, and the resulting composite structure is cured.
U.S. Pat. No. 5,928,736 issued to Parekh teaches a composite structure having an integrated aperture, and a method for its preparation. The composite structure is fabricated by providing a mandrel having a winding surface and a shaped plug placed into the winding surface such that it extends outwardly. An uncured composite material is wound about the winding surface typical of the filament winding process. The addition of a shaped plug provides a means to avoid any cutting or machining that may be required in order to form an aperture therein. In the winding process, the fibre is wound about the mandrel and around the shaped plug such that the fibres are clustered around the edges of the plug. The cluster of fibres results in an increase in strength in the thickened region around the periphery of the aperture.
In accordance with one aspect of the present invention, there is provided a polygonal composite structure formed on a mandrel having a longitudinal axis running lengthwise along the composite structure. The composite structure having two structural layers, and a plurality of corner pieces equal to the number of sides of the composite structure. Each of the plurality of the corner pieces is oriented lengthwise along the polygonal composite structure and is located at each corner of the polygonal composite structure. A corner is defined by the intersection of adjacent sides of the composite structure. The two structural layers comprise an inner structural layer and an outer structural layer each formed by winding a fibre reinforcement material about the mandrel. The fibre reinforcement material has a resin applied thereon and is wound about the mandrel at a winding angle relative to the longitudinal axis of the mandrel. The resin is selected from the group consisting of epoxy, polyester, vinylester resin, and combinations thereof. The fibre reinforcement is selected from the group consisting of glass fibre, carbon fibre, synthetic fibres, and combinations thereof. Each of the plurality of corner pieces is located between the inner structural layer and outer structural layer such that the placement of the corner pieces actually defines the two structural layers at each corer. When in place, the corner pieces define the shape of each corner. On winding of the fibre reinforcement material about the mandrel, placement of the corner pieces enables the forming of a sharper corner and enables a more even wall thickness. The geometric shape of the mandrel further defines the geometric shape of the composite structure formed on the winding of the fibre reinforcement material about the mandrel. The geometric shape of the mandrel represents the desired resultant shape of the composite structure.
The fibre reinforcement material forming a composite structure is at least a single band of fibre. This single band is formed from two or more thinner strands of fibre which are intertwined together and impregnated by a resin. Typically, multiple strands are used to form a band of filament and that band is impregnated with resin and is then wound about the mandrel. The number of winding wraps of the band about the mandrel creates a winding pattern where the pattern is designed to cover the entire surface uniformly with the fibres.
In the first embodiment the polygonal composite structure is rectangular, however, the composite structure may also be square. Further, the shape of the composite structure is influenced by the desired structural properties. Generally, the polygonal composite structure has six or fewer sides, usually four sides.
In any event, a method of manufacture of this system in keeping with the present invention may be described in terms of a series of steps, those steps being defined as follows:
(a) The mandrel having a longitudinal axis and a winding surface, is placed in a position such that fibre reinforcement material may be wound onto the winding surface.
(b) Fibre reinforcement material having a resin applied thereon is wound about the mandrel so as to form an inner structural layer.
(c) At least one of the plurality of corner pieces is placed along an edge of the composite structure at a corner of the composite structure.
(d) A portion of the fibre reinforcement is wound about the mandrel, where the portion wound, is at least enough to hold the at least one corner piece applied in the correct position.
(e) Steps (c) and (d) are repeated until each of the plurality of the corner pieces is placed at a corner of the composite structure, such that each corner of the composite structure has a corresponding corner piece applied thereon.
(f) The fibre reinforcement material having a resin applied thereon is wound about the mandrel so as to form an outer structural layer.
(g) The composite structure is left to cure for a period of time. Each of the plurality of corner pieces applied in steps (c) and (d) is located between the inner structural layer and the outer structural layer, where the layers are defined by the placement of the corner pieces.
The method of making a composite structure may further include step (h) which would occur following step (f). The curing process described in step (g) would occur while the composite structure is in a pressure mould. Step (h) therefore being:
(h) Placing the composite structure in a pressure mould such that pressure is evenly distributed on all faces of the composite structure. The pressure mould applies an evenly distributed force such that the composite structure is compressed to predetermined dimensions. The dimensions are influenced by the desired structural strength of the composite structure.
The method of making the composite structure as described above may further include step (i) which occurs after step (g). Step (i) being:
(i) Removal of the mandrel by withdrawing the mandrel away from the composite structure.
Typically the removal of the mandrel is by pulling the mandrel away from the composite structure.
The placement of each of the plurality of corner pieces along the length of the composite structure, at each of the corners of the composite structure, is typically measured. The corner pieces generally comprise a first arm and a second arm. The joint between the first and second arms defines the corner and, in turn, an angle. The geometry of each of the corner pieces is generally determined in the initial drawings of the composite structure. The geometry is directly related to the requirements of the finished dimensions including wall thickness, corner radius, and in turn, the desired structural strength of the composite structure. The angle formed by one of the plurality of the corner pieces is equivalent to at least one of an interior angle of the mandrel. The number of interior angles of the mandrel is directly related to the geometric shape of the mandrel.
The novel features which are believed to be characteristic of the present invention, as to its structure, organization, use and method of operation, together with further objectives and advantages thereof, will be better understood from the following drawings in which a presently preferred embodiment of the invention will now be illustrated by way of example. It is expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. Embodiments of this invention will now be described by way of example in association with the accompanying drawings in which:
FIG. 1 is a cross sectional view of a composite beam typical of the prior art;
FIG. 2 is a cross sectional view, also detailing a composite beam typical of the prior art;
FIG. 3 is a cross sectional view of a first embodiment of a polygonal composite structure of the present invention;
FIG. 4 is a cross sectional view of the first embodiment of a polygonal composite structure of the present invention, with the mandrel removed;
FIG. 5 is an enlarged view of a corner portion of FIG. 4; and
FIG. 6 is a side view of the first embodiment of a polygonal composite structure with the mandrel still in place.